It is well known that, under excitation, a semiconductor device will emit a very small amount of light. However, the light emission process in silicon is notable for its poor quantum efficiency. Less than 0.01% of whole electron pairs recombine by emitting a photon. Therefore, it would require hundreds of milliamperes of current through a silicon device in avalanche breakdown to view the light emission with the human eye. As shown in the block diagram of FIG. 1, it is also well known to focus the light emitted using an optical microscope on a microchannel intensifier. The objective of taking these focusing and intensifying steps is to develop an image of the location of hot electron emission and other impact itemization from a semiconductor device over a period of time. In this way, transistors subject to hot electron effects may be identified.
Such hot electron effects have been recognized as a problem in small scale semiconductor devices for a long time. Progressive scaling of the feature sizes of integrated circuit technology has made new products much more susceptible to such hot electron effects. As explained in the article, "Analysis of Product Hot Electron Problems by Gated Emission Spectroscopy" by N. Khurana and C. L. Chiang (CITATION), the need exists for an analytical instrument allowing the examination of a product for rapid determination of the transistors most vulnerable to hot electron effects. The article and a related patent U.S. Pat. No. 4,680,635 disclose a system of the type shown in FIG. 1 of the present application. That system uses a microchannel intensifier 10 which projects the image of the semiconductor device on the photocathode of an image tube creating photoelectrons in proportion to the intensity of the optical image. These photoelectrons are accelerated and focused by the electron optics onto a phosphorous screen. Each photoelectron generates several hundred photons striking the phosphor, thus providing a much brighter output image which can in turn be conveyed to a solid state camera 14 to generate the desired television signal to be used to drive an image processor 15 which in turn displays its output on a television monitor 6. In this way, the light which, as emitted, is much fainter than the human eye can see, can be amplified. The objective is to render it visible on a television monitor display, and have the light sufficiently focused so the particular point on the semiconductor device emitting the light (and therefore presumably subject to hot electron effects) can be identified. Thus, the light emitted indicates stress points and defects on silicon chips that are too small to detect with the naked eye or with optical microscopes alone.
The problem arises that typically, defects sought to be detected emit a very limited number of photons, perhaps only one photon every three seconds. Therefore, if a typical processing cycle is one second, the result will be flicker or noise. Thus, while the prior art amplifies the light, it makes it brighter but does not solve the noise problem.
The classic solution this problem as shown in FIG. 2 has been integration over time, taking the light inputs over a period of, perhaps, 30 seconds, whereby the average number of photons over each time period can be made more consistent. Such integration, which uses a standard integrator 20 to feed the enhancement device 22 suffers from a deficiency in requiring a considerable amount of time to process the image, i.e., five seconds to two minutes of image integration followed by typically five seconds of enhancing.
Another disadvantage of this approach is that the duration of integration required to raise the information signals to useful strength depends on the intensity of the signal output from the defective chip region, which is not known a priori. The proper integration time can be guessed only after viewing the processed image. If a user underestimates integration time, the process will have to be repeated. Typically, the integration time is overestimated, which is wasteful of the tester's time.
Even given this integration, however, it is then necessary to do processing to reduce the noise level, specifically background subtraction. This processing step s necessary because of the fact that some thermal emission occurs even in the absence of light emission, which will be detected by this extremely sensitive system. With integration over time, this background can be significant and would represent errors in the absence of some noise processing. The difficulty presented in subtraction of the background noise is that the level of background noise will fluctuate, i.e., it is not a constant. Therefore, subtraction of background noise is not so simple as setting a fixed threshold level below which the information is eliminated from consideration. I is this fluctuation in the background noise, therefore, that limits the sensitivity of systems disclosed in the prior art.
Another difficulty with known systems is that all the elements of the system must be incorporated within a "black box," with absolutely no local sources of illumination which can interfere with the light being detected from the emitting source. This makes wafer handling both difficult and slow.
A further difficulty with the mechanics of known prior art systems is that in reflecting the light to follow the necessary optical path from the wafer under test to the eye piece, mirrors are used, which typically only reflect 50% of the light, thereby losing up to one-half of the light emitted from the device under test.